Diff but not when 0

I need a way to do what diff does but ignore differences when the upper bound or lower bound is 0;

X = [1 1 2 0 5 0 13 21];

Y = diff(X)

Y =

     0     1     -2     5    -5     13     8

X = [1 1 2 0 5 0 13 21];

Y = the_diff_I_Need(X)

Y =

     0     1     0     0     0    0     8

Efficiency is important here.

Thanks

Answer 1

[dpb]

X(X==0)=nan;     % flag zero locations
d=diff(X);       % compute difference w/ flagged
d(isnan(d))=0;   % replace NaN in output if desired

For the sample given

>> Y=X;Y(Y==0)=nan;
>> d=diff(Y); d(isnan(d))=0
d =
   0     1     0     0     0     0     8
>>